U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF TERRANES IN THE CENTRAL VIRGINIA PIEDMONT

New U-Pb detrital zircon (DZ) age data yield insight on Paleozoic orogenesis in the central VA Piedmont. Recent mapping near Louisa, VA suggests that there is a distinct assemblage of rocks bounded to the SE by the Ord. Chopawamsic Formation (CF) and to the NW by the Mine Run Complex (MRC) of the Laurentian affinity Potomac terrane. Here, a ~1-2 km wide belt includes metagraywacke and blocks of ultramafic rocks. Published DZ data (Hughes et al., 2014) from rocks in Storck, VA, ~60 km NE of Louisa, yield ages of 533-2000+ Ma that suggest a peri-Gondwanan source, which is in contrast with DZ data from adjoining CF and MRC rocks at that latitude. Hughes et al. (2015) suggest that this zone represents the closure of the Iapetus basin during the accretion of the Chopawamsic volcanic arc to Laurentia in the Ord. Our new DZ data from the potentially correlative belt near Louisa contains ages of ~900-2800 Ma--notably lacking Ediacaran-Eocambrian ages, but otherwise consistent with DZ ages from the rocks in Storck and rocks along strike to the north in the Potomac Terrane (Martin et al. 2015). Neither a distinctly peri-Gondwanan or Laurentian source is unequivocal in our sample; rather it may represent derivation from both.

U-Pb DZ ages from basal quartzite of the Ord. Quantico Formation (QF) were also determined. Horton et al. (2010) report a U-Pb zircon crystallization age of ~448±4 Ma for a dominantly volcaniclastic rock from the base of the QF in northern VA. Hughes et al. (2014) interpreted the QF as a Late Ord.-Sil.successor basin deposited unconformably above the CF and after the 444-437 Ma emplacement of the Ellisville pluton. Our DZ data from the QF contains a pronounced age peak at 450 Ma. However, there are ~10 analyses of DZ that are <440 Ma and as young as 427 Ma. Our data along with Hughes et al. (2014) data from the Bremo Member of the Arvonia Formation (AF) (successor basin SW of QF), suggest that the QF/AF is a successor basin sequence. A successor basin model potentially requires the basal QF volcaniclastic rocks dated by Horton et al. (2010) to be secondarily derived and recycled almost exclusively from a nearby source of ~448 Ma volcanic rocks, perhaps from the underlying CF. Both our QF sample and the Bremo Member-AF sample of Hughes et al. (2014) have age peaks at ~450 Ma, which suggests an abundance of Late Ord.-age zircon recycled in the area.